Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants are composed of gaseous compounds, such as the oxides of nitrogen (NOX). Due to increased awareness of the environment, exhaust emission standards have become more stringent and the amount of NOX emitted from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. In order to ensure compliance with the regulation of these compounds, some engine manufacturers have implemented a strategy called Selective Catalytic Reduction (SCR).
SCR is a process where gaseous or liquid reductant (most commonly a solution of urea solid and water) is added to the exhaust gas stream of an engine and adsorbed onto a downstream catalyst. The reductant decomposes into ammonia (NH3), which reacts with NOX in the exhaust gas to form H2O and N2 that can be safely released to the atmosphere. Although SCR can be an effective method for reducing NOX, the introduction of reductant into the exhaust gas stream can be problematic in some situations. For example, when the reductant is introduced into a relatively cool portion of an exhaust passage, byproducts of the reductant such as cyanuric acid, biuret, and melamine can be deposited on walls of the passage near an introduction site. These deposits can cause exhaust flow and/or reductant blockage, which can lead to degradation of the NOX conversion process and engine performance.
One attempt to reduce deposit buildup associated with the SCR process is described in European Patent Specification 1,781,908 of Kuenkel et al. that published on Mar. 31, 2010 (the '908 publication). The '908 publication discloses an arrangement for supplying a reducing medium into an exhaust gas of an internal combustion engine. The arrangement includes an exhaust line and a tubular element fitted inside the exhaust line to create a first exhaust passage through the tubular element and a second exhaust passage between an outer surface of the tubular element and an inner surface of the exhaust line. The tubular element is made of a material having good heat conducting properties such that heat from the second exhaust passage is transferred to the tubular element to insulate the tubular element from a cooling action of the environment and to maintain the tubular element at a vaporization point of the medium injected into the first exhaust passage. In an alternative embodiment, the tubular element is omitted and a portion of the exhaust line at the reductant injection location is instead fabricated from a material having high thermally insulating properties to reduce the cooling action of the environment at the injection site.
Although perhaps somewhat effective at reducing the formation of reductant deposits, the system of the '908 publication may still be less than optimal. In particular, the tubular element of the '908 publication extends only downstream from the reductant introduction site. In this embodiment, the entire tubular element may be continuously cooled by the introduction of reductant and only a limited amount of heat may be transferred into the tubular element. In the alternative embodiment, only insulation is relied on to inhibit deposit formation, which may be insufficient under some conditions.
The exhaust system of the present disclosure solves one or more of the problems set forth above and/or other problems of the prior art.